Golf ball

ABSTRACT

The present invention provides a golf ball that achieves a satisfactorily long carry distance when hit with a driver and that accurately generates a desirable amount of backspin in an approach shot. The golf ball of the present invention has a spherical main part, a plurality of ribs formed on the surface of the main part, an interlayer that is placed in depressions surrounded by the ribs and has a hardness greater than that of the ribs, an inner cover that covers the interlayer, and an outer cover that covers the inner cover and has a hardness lower than the inner cover.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a multi-layered golf ball.

(2) Description of the Related Art

Recently, several kinds of golf balls exhibiting both high reboundresilience and a soft feel when hit have been proposed. One example ofsuch a golf ball is a multi-layered golf ball in which the ball iscomposed of a plurality of layers. For example, WO2005/089883 (PatentDocument 1: Domestic re-publication of PCT international application)discloses a golf ball having ribs provided on the surface of a sphericalmain part, and an interlayer formed in the depressions surrounded by theribs. By setting the hardness of the interlayer higher than that of theribs, the golf ball described above achieves the following effects. Whena golf club comes into contact with a golf ball, usually the ball isdeformed in the circumferential direction due to friction generatedbetween the ball and the clubface. When the deformed ball returns to itsoriginal condition due to elastic resistance, a force in the directionopposite to the direction of backspin is applied to the ball. At thismoment, the greater the deformation of the deformed ball, the more thebackspin is suppressed, and the longer the carry distance becomes.

In the golf ball of Patent Document 1, the ribs enhance the elasticresistance, which is the force applied when the ball is returning to itsoriginal condition, and therefore the backspin, can be effectivelyreduced. More specifically, in this golf ball, because the hardness ofthe ribs is lower than that of the interlayer, the ribs deform to agreater degree than the interlayer. The ribs are not mere protrusionsbut are structured so as to form walls surrounding the interlayer, andtherefore when the ribs are returning to their original condition, theforce of the entire wall strongly acts on the interlayer from theperimeter of the interlayer, and this increases the force opposing thebackspin. As a result, a significantly longer carry distance can beachieved. This effect is particularly remarkable when the ball is hit bya driver, etc., which is designed to attain a long carry distance.

The golf ball described above is desirable for hitting with a driver.However, it has a drawback when it is hit with an iron in an approachshot. In an approach shot, it is important to stop the ball by usingbackspin; however, the ball cannot be accurately stopped when the forceopposing the backspin is increased as described above.

An object of the present invention is to provide a golf ball with whicha long carry distance can be achieved when hit by a driver, while alsoenabling backspin to be accurately generated for an approach shot.

BRIEF SUMMARY OF THE INVENTION

The golf ball of the present invention comprises a spherical main part,a plurality of ribs formed on the surface of the main part, aninterlayer that is harder than the ribs and placed in depressionssurrounded by the ribs, an inner cover that covers the interlayer, andan outer cover that is softer than the inner cover and that covers theinner cover.

In this structure, because the hardness of the ribs is lower than thatof the interlayer, the ribs deform to a greater degree than theinterlayer when hit. The ribs are not mere protrusions but arestructured so as to form walls surrounding the interlayer, and thereforewhen the ribs are returning to their original condition, the force ofthe entire wall strongly acts on the interlayer from the perimeter ofthe interlayer, and this increases the force opposing the backspin. As aresult, a significantly longer carry distance can be achieved. Thiseffect is particularly remarkable when the ball is hit by a driver,etc., which is designed to attain a long carry distance.

Because the inner cover is provided between the interlayer and the outercover, direct transmission of the striking force to the interlayer andthe ribs can be prevented in an approach shot, such as one performedusing an iron, in which the deformation of the ball is small. As aresult, the increase of the force opposing the backspin can beprevented. Therefore, the golf ball of the present invention achieves along carry distance by reducing the amount of backspin when hit by adriver. Furthermore, the golf ball of the present invention can beaccurately stopped by the application of backspin when it is hit by aniron.

Because the hardness of the outer cover is lower than that of the innercover, a soft feeling can be obtained when hit. Furthermore, becausethis structure can increase the deformation of the outer cover, thebackspin effect can be increased when hit by an iron. Because thehardness of the inner cover is greater than that of the outer cover inthis embodiment, even though the outer cover has a soft feeling whenhit, a high rebound resilience can be attained by the inner cover. Thisachieves a longer carry distance. The rebound resilience can beincreased by setting the Shore D hardness of the inner cover, forexample, to 55 to 70. A soft feeling when hit can be obtained by settingthe Shore D hardness of the outer cover to 54 to 60.

In the golf ball described above, the inner cover can be made harderthan the interlayer. This makes the hardness of the inner portion in theradical direction lower than that of the inner cover, reducing thehitting hardness attributable to the inner cover.

In the golf ball described above, the total thickness of the inner coverand the outer cover can be 1.9 mm or more. The deformation of the ballin an approach shot using a wedge may sometimes reach about 2.0 mm.Having the total thickness of the inner cover and the outer cover 1.9 mmor more makes it difficult for the hitting impact to be transmitted tothe interlayer and ribs when making an approach shot using a wedge. Thismakes it possible to effectively reduce the backspin-canceling effect ofthe ribs. Therefore, the ball can be accurately stopped in an approachshot using a wedge. Furthermore, having such a total thickness can alsoimprove the durability of the ball.

In the golf ball described above, the hardness of the main part can bemade the same as that of the ribs by forming the main part and the ribsas a unit. In this structure, the hardness of the interlayer is not onlygreater than that of the ribs but is also greater than that of the mainpart, so the amount of spin can be reduced to obtain a longer carrydistance.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view showing a golf ball according to afirst embodiment of the present invention.

FIG. 2 is a perspective view of the core of the golf ball of FIG. 1.

FIG. 3 is a cross-sectional view showing the condition of the golf ballof FIG. 1 when hit.

FIG. 4 is a perspective view of another example of the core of the golfball of FIG. 1.

FIG. 5 is a cross-sectional view of another example of the core of FIG.1.

FIG. 6 is a cross-sectional view of another example of the core of FIG.1.

FIG. 7 is a cross-sectional view of another example of the core of FIG.1.

FIG. 8 is a cross-sectional view of another example of the core of FIG.1.

FIG. 9 is a cross-sectional view of another example of the core of FIG.1.

FIG. 10 illustrates an example of a process for forming the golf ball ofFIG. 1.

FIG. 11 illustrates an example of a process for forming the golf ball ofFIG. 1.

EXPLANATION OF NUMERICAL SYMBOLS

-   1 golf ball-   3 core-   5 interlayer-   7 inner cover-   9 main part-   11 rib-   13 depressions-   15 outer cover

DESCRIPTION OF EMBODIMENTS

One embodiment of the multi-piece golf ball of the present invention isexplained below with reference to the drawings. FIG. 1 is across-sectional view of the golf ball according to the presentembodiment.

As shown in FIG. 1, the golf ball 1 of the present embodiment is amulti-piece golf ball comprising a core 3 covered with an interlayer 5,an inner cover and an outer cover 15. According to the rules (see R&Aand USGA), the diameter of a golf ball must be not less than 42.67 mm.However, taking aerodynamic characteristics and the like intoconsideration, it is preferable that the diameter of the ball be assmall as possible. Therefore, it can be, for example, 42.7 mm to 42.9mm. The core 3 is composed of a rubber composition, and, as shown inFIG. 2, comprises a spherical main part 9 and three ribs (protrusions)11 molded as a unit on the surface of the spherical main part 9. Eachrib 11 extends along one of the great circles drawn around the main part9 so as to intersect each other at right angles. These ribs form eightdepressions 13 on the surface of the main part 9.

It is preferable that the diameter of the main part 9 be 29.5 to 36.7mm, and more preferably 29.9 to 35.1 mm. The height of the ribs 11 ispreferably 1.0 to 4.0 mm, and more preferably 1.2 to 2.0 mm. The Shore Dhardness of the surface of the core 3 is preferably 50 to 60, and morepreferably 53 to 57. When the Shore D hardness thereof is lower than 50,the rebound resilience becomes unduly low, resulting in a squishy feel.In contrast, when the Shore D hardness exceeds 60, the ball becomes toohard, resulting in a poor impact feel.

As shown in FIGS. 1 and 2, each rib 11 is structured so as to have atrapezoidal profile in its sideways cross-section in such a manner thatits width increases as it approaches the main part 9. It is preferablethat the width of the top portion a of each rib in the outward radialdirection be 1.5 to 2.5 mm and that the width of the bottom portion b inthe inward radial direction be 3.0 to 6.0 mm. The widths of the endportions of the rib 11 may be set outside this range; however, bysetting a lower limit for the width of each end portion of the rib 11,it is possible to prevent the rib 11 from being deformed by the pressureof filling the interlayer 5 that results from tightly closing the moldwhen filling the material for the interlayer 5 during the manufacturingprocess. As a result, it is possible to accurately hold the core 9 inthe center of the mold.

The interlayer 5 is made of a rubber composition or an elastomer, coversthe surface of the core 3, and has a substantially spherical outsideshape. As shown in FIG. 1, the interlayer 5 has almost the samethickness as the height of the ribs 11, and fills in each of the eightdepressions 13 surrounded by the ribs 11. The top portions of the ribs11 are exposed through the surface of the interlayer 5. To controlbackspin, as described later, it is necessary to make the hardness ofthe interlayer 5 greater than that of the core 3. The Shore D hardnessof the interlayer 5 is preferably 53 to 62, and more preferably 56 to60. In this structure, the Shore D hardness of the interlayer 5 isgreater than that of the core 3 preferably by 1 to 5.

The inner cover 7 is composed of an elastomer, and covers the topportions of the ribs 11 and the interlayer 5. It is preferable that thethickness of the inner cover 7 be 0.9 to 1.7 mm, and more preferably 0.9to 1.5 mm. The thickness of the inner cover 7 may be set outside thisrange; however, if the thickness of the inner cover 7 is less than 0.9mm, the ball becomes too soft, reducing the rebound resilience and thedurability of the ball. On the other hand, if it exceeds 1.7 mm, theimpact feel becomes too hard. The Shore D hardness on the surface of theinner cover 7 is preferably 55 to 70, more preferably 58 to 68, andstill more preferably 64 to 68.

The outer cover 15 is composed of an elastomer, and covers the innercover 7. Predetermined dimples (not shown) are formed on the outersurface of the outer cover. The thickness of the outer cover 15 ispreferably 0.8 to 1.3 mm, and more preferably 0.9 to 1.2 mm. Thethickness of the outer cover 15 may be set outside this range; however,if the thickness of the outer cover 15 is less than 0.8 mm, thedurability of the outer cover 15 decreases remarkably and moldingbecomes difficult. On the other hand, if it exceeds 1.3 mm, the reboundresilience is excessively reduced and a satisfactory carry distancecannot be obtained. The outer cover 15 has a Shore D hardness on thegolf ball surface of preferably 54 to 60, and more preferably 56 to 60.The thickness of the outer cover 15 is defined as the distance from anarbitrary point on the outermost part in the outward radial directionwhere no dimple is formed to an arbitrary point that comes into contactwith the interlayer that is measured along the normal line. The totalthickness of the inner cover 7 and the outer cover 15 is preferably 1.9to 3.0 mm, more preferably 2.0 to 2.8 mm, and still more preferably 2.0to 2.6 mm. Setting the total thickness within this range enables apreferable rebound resilience and durability to be attained.

The dimples formed in the outer cover 15 are explained below. Thedimples can be circular, polygonal, oval or a like shape, and one typeor a combination of two or more types can be used. For example, circulardimples having a diameter of 2.5 to 4.5 mm may be provided. The numberof dimples is 350 to 450, and preferably 360 to 410. If too many dimplesare provided, the trajectory of the ball lowers, and this may reduce thecarry distance. On the other hand, if the number of dimples is toosmall, the trajectory of the ball rises, and this may also reduce thecarry distance. The proportion of the area of the dimples relative tothe total area of the spherical surface of the golf ball is preferably73% or more, and more preferably 75% or more.

The component materials of the above-described golf ball are explainedin detail. The core 3 can be manufactured using a known rubbercomposition comprising a base rubber, a cross-linking agent, anunsaturated carboxylic acid metal salt, a filler, etc. Specific examplesof the base rubber include natural rubber, polyisobutylene rubber,styrenebutadiene rubber, EPDM, etc. Among these, it is preferable to usehigh-cis polybutadiene that contains 80% or more cis-1,4 bonds.

Specific examples of cross-linking agents include dicumyl peroxide,t-butylperoxide, and like organic peroxides; however, it is particularlypreferable to use dicumyl peroxide. The compounding ratio of thecross-linking agent is generally 0.3 to 5 parts by weight, andpreferably 0.5 to 2 parts by weight per 100 parts by weight of the baserubber.

As metal salts of unsaturated carboxylic acids, it is preferable to usemonovalent or bivalent metal salts of acrylic acid, methacrylic acid,and like C₃ to C₈ unsaturated carboxylic acids. Among these, the use ofzinc acrylate can improve the rebound resilience of the ball and isparticularly preferable. The compounding ratio of the unsaturatedcarboxylic acid metal salt is preferably 10 to 40 parts by weight per100 parts by weight of base rubber.

Examples of the fillers include those generally added to cores. Specificexamples thereof include zinc oxide, barium sulfate, calcium carbonate,etc. The preferable compounding ratio of the filler is 2 to 50 parts byweight per 100 parts by weight of base rubber. If necessary, it is alsopossible to add an antioxidant, a peptizer, and the like.

The interlayer 5 is composed of a rubber composition or elastomer asdescribed above. When a rubber composition is used, the same materialsas used for the core 3 described above can be used. However, it ispreferable that the compounding ratio of unsaturated carboxylic acids beincreased to make the interlayer harder than the core 3.

When the interlayer 5 is composed of an elastomer, it is possible touse, for example, a styrene/butadiene/styrene block copolymer (SBS), astyrene/isoprene/styrene block copolymer (SIS), astyrene/ethylene/butylene/styrene block copolymer (SEBS), astyrene/ethylene/propylene/styrene block copolymer (SEPS), or likestyrene-based thermoplastic elastomer; an olefin-based thermoplasticelastomer having polyethylene or polypropylene as a hard segment andbutadiene rubber, acrylonitrile butadiene rubber or ethylene/propylenerubber as a soft segment; a vinyl chloride-based plastic elastomerhaving crystallized poly(vinyl chloride) as a hard segment and amorphouspoly (vinyl chloride) or an acrylonitrile butadiene rubber as a softsegment; a urethane-based plastic elastomer having polyurethane as ahard segment and polyether or polyester urethane as a soft segment; apolyester based plastic elastomer having polyester as a hard segment andpolyether or polyester as a soft segment; an polyamide based plasticelastomer having polyamide as a hard segment and polyether or polyesteras a soft segment; an ionomer resin, etc.

Each of the inner cover 7 and the outer cover 15 is composed of a knownelastomer. Examples of usable elastomers are the same as those used informing the interlayer described above. Among these, ionomer resin ispreferable in obtaining desirable rebound resilience, durability,moldability and the like. Examples of usable ionomer resins includeHimilan 1706, Himilan 1605 (manufactured by Mitsui-DuPont PolychemicalsCo., Ltd.), Surlyn 9910, Surlyn 8940, Surlyn 8150, Surlyn 8120, andSurlyn 8320 (manufactured by DuPont). For example, HPF1000 and HPF2000(ionomers manufactured by DuPont) are preferably used as the materialfor the inner cover 7 from the viewpoint of a soft feel and reboundresilience. For example, HPC AD1043 (an ionomer manufactured by DuPont)is preferably used as the material for the outer cover 15 from theviewpoint of rebound resilience, scratch resistance and the like.

These materials may be used singly or in combination to improve theirperformance.

As described above, the preset embodiment achieves the following effect.Usually, when a golf club comes into contact with a golf ball, the ballis deformed in the circumferential direction due to friction generatedbetween the ball and the clubface. As the deformed ball proceeds toresume its original condition due to elastic resistance, a force in thedirection opposite to the backspin direction is applied to the ball. Atthis time, the greater the ball deformation, the stronger the backspinsuppression, and the longer the carry distance.

In the golf ball of the present embodiment, the ribs 11 enhance theelastic resistance that acts to return the ball to its originalcondition, so the backspin can be effectively reduced. Morespecifically, as shown in FIG. 3( a), in this golf ball, because thehardness of the ribs 11 is lower than that of the interlayer 5, when theball is hit by a club C, the ribs 11 deform more severely than theinterlayer 5. Due to the impact, a force causing backspin B is appliedto the ball. When the ball separates from the club C, as shown in FIG.3( b), the low-hardness rib 11 returns to its original condition fromthe deformed condition, and this restoration applies a force F to theball in the direction that cancels the backspin B. As a result, the spinamount is reduced and a longer carry distance is attained. Inparticular, in the present embodiment, the ribs 11 are not simpleprotrusions but are structured so as to form walls surrounding theinterlayer 5, and therefore when the ribs 11 are returning to theiroriginal condition, the force of the entire wall greatly acts on theinterlayer 5 from the perimeter of the interlayer 5, and this increasesthe force F opposite to the backspin B. As a result, the amount ofbackspin is reduced and a significantly longer carry distance isachieved. This effect is particularly remarkable when the ball is hit bya driver, etc., which is designed to obtain a long carry distance. InFIG. 3, the current condition is shown by the solid lines and thecondition immediately before the current condition is shown by thedashed lines.

However, because an inner cover 7 is provided between the interlayer 5and the outer cover 15, transmission of the striking force to theinterlayer 5 and the ribs 11 can be controlled in an approach shot, suchas one performed using an iron, in which the ball deformation is small.As a result, the increase of the above-described force canceling thebackspin can be prevented. Therefore, the golf ball of the presentinvention achieves a long carry distance by reducing the amount ofbackspin when hit by a driver. Furthermore, the golf ball of the presentinvention can be accurately stopped by the application of backspin whenhit by an iron.

Because the hardness of the outer cover 15 is lower than that of theinner cover 7, a soft feeling can be obtained when hit. Furthermore,because the hardness of the outer cover 15 is low, the outer cover 15 iseasily deformed. In other words, because the outer cover 15, which isthe outermost cover of the ball, is easily deformed, the backspin effectcan be increased when hit by an iron.

Also, because the hardness of the inner cover 7 is high, even though theouter cover 15 causes a soft feeling when hit, the rebound resiliencecan be improved by the inner cover 7. This makes it possible to achievea high rebound resilience even when the club head speed is slow, whichenables a longer carry distance.

As described above, by providing the ribs 11 on the core 3 and settingthe hardness of the interlayer 5 greater than that of the ribs 11, thegolf ball of the present embodiment achieves a backspin reductioneffect. The extent of this backspin reduction effect can be controlledby adjusting the height of the ribs 11, and the difference in thehardness between the interlayer 5 and the ribs 11. This makes itpossible to desirably control the carry distance. The carry distance isalso controllable by adjusting the lift force applied to the ball bysuitably selecting the shape of the dimples and the like. The adjustmentof the ball performance is conducted according to the user's level anddesired performance. For example, among professional and other top-classgolfers, there is a demand for a ball in which, when the golfer wishesto stop the ball in an approach shot, the backspin reduction effect dueto the ribs is suppressed so that a certain amount of backspin can begenerated. However, if too much focus is placed on approach shots, theball tends to rise excessively when hit by a driver, which shortens thecarry distance. This drawback can be overcome by selecting a design forthe dimples and the like that prevents the ball from rising excessivelyby reducing the lift force applied to the ball, so that a longer carrydistance can be achieved when hit by a driver.

In response to this demand, the lift force may be set as shown below.For example, immediately after being hit by a driver, a ball has avelocity of 62 m/s, a spin rate of 2,400 rpm, and a spin parameter of0.09. In this case, the lift coefficient is preferably 0.13 to 0.17.Immediately after being hit by an iron, a ball has, for example, avelocity of 46 m/s, a spin rate of 4,700 rpm, and a spin parameter of0.23. However, when hit from the rough, a ball has a spin rate of about2,500 rpm, which is remarkably lower than when the ball is hit from thefairway. Here, the spin parameter becomes 0.12. When the spin parameteris 0.12, the lift coefficient is preferably 0.16 to 0.20. As describedabove, having an unduly high lift coefficient results in an unexpectedcarry distance caused by excessive rising and the like. In contrast, ifthe lift coefficient is too small, the ball trajectory and launch anglebecome too low when hit from the rough, and the ball does not stop asdesired. Based on the structure of the ball described above, it ispreferable to design the dimples and the like so as to obtain the liftforce described above.

The force applied to a ball, such as the aforementioned lift force, canbe expressed by the trajectory equation shown below.

F=FL+FD+Mg

F: Force applied to the golf ball

FL: Lift force (N)

FD: Drag force (N)

M: Mass of the golf ball (kg)

g: Gravitational acceleration (m/s²)

The lift force (FL) and drag force (FD) can be expressed by theequations shown below.

FL=0.5×CL×ρ×A×V ²

FD=0.5×CD×ρ×A×V ²

CL: Lift coefficient

CD: Drag coefficient

ρ: Air density (kg/m³)

A: Cross-sectional area of the golf ball (m²)

V: Velocity of the golf ball (m/s)

The spin parameter (Sp) can be expressed by the equation shown below.

Sp=π×d×N/V

d: Diameter of the golf ball (m)

N: Rate of rotation of the golf ball (rps)

The spin parameter, lift force and the like can be measured by trackinga ball shot by a golf robot using TrackMan (manufactured by InteractiveSports Games Co., Ltd.). TrackMan is a system designed for tracking andmeasuring ball flight using Doppler radar technology.

The above-described ribs may be formed into various shapes; however,from the viewpoint of effectively molding the interlayer, it ispreferable to provide a notch in the rib having a structure as describedbelow. FIG. 4 is a perspective view of a core provided with notches.FIG. 5 is a cross-sectional view of the core of FIG. 4. As shown inFIGS. 4 and 5, the notch 24 is structured so as to have a bottom surface24 a extending along a tangent plane H that passes through theintersection P of the great circles. In other words, the notch 24 isformed by excising the rib 11 at the tangent plane H. By forming notches24 in this manner, four depressions 13 that are arranged so as to have acommon center at an intersection P of the great circles are made tocommunicate with each other, and the material for the interlayer canreadily spread between the depressions 13 via the notches 24. In thiscase, as shown in FIG. 6, it is also possible to form the bottom surface24 a of the notch 24 along a plane H₁ that extends away from the tangentplane H by being slanted toward the center of the rib 11 by 1 to 3°,i.e., a plane having an angle made between the normal line n of the mainpart 9 passing through the intersection P is 91 to 93° as viewed fromthe front. This arrangement enables the angle to serve as a draft angle,and, for example, when a core is molded using two molds, such as anupper mold and a lower mold, the core 3 can be easily removed from themold.

When the notch 24 is formed as described above, it is preferable thatthe length of the notchless top portion of each arc section S of theribs in the arc direction, divided at the intersection P, as shown inFIG. 5, be no smaller than 10 mm.

As shown in FIG. 7, it is also possible to form a notch 24 so as to havea bottom surface 24 a extending along a plane H₂ that is perpendicularto the normal line n that passes through the mid point in the heightdirection of the rib 11. In this case, in order to smoothly spread theinterlayer material throughout the depressions 13, it is preferable thatthe distance D from the top portion of the virtual rib 11 without anotch 24 to the bottom surface 24 a be no less than 1.2 mm. The length Lshould preferably be no less than 10 mm, as in the above-described case.Furthermore, it is possible to form a draft angle by forming the bottomsurface 24 a of the notch 24 along a plane that has an angle of 91 to93° relative to the normal line n in the same manner as shown in FIG. 6.

It is also possible to provide a notch in the middle of the arc sectionS of the rib 11 in the arc direction. As shown in FIG. 8( a), it ispossible to form the notch 25 so as to have two bottom surfaces 25 aeach extending toward the intersection P from a point on the normal linem of the main body 9 that passes through the midpoint Q of each arcsection in the circumferential direction. In this case, it is preferablethat the angle made between the bottom surfaces 25 a and the normal linem be 45 to 48° as viewed from the front. This arrangement makes itpossible to easily remove the core 3 from the mold. However, if thisangle exceeds 48°, the above-described length L of the rib in thecircumferential direction becomes unduly short. It is preferable thatthe depth D of the notch 25 be no less than 1.2 mm. The depth D may beset outside of this range; however, by setting the depth D in thisrange, it is possible to smoothly spread the interlayer materialthroughout the depressions 13. Note that the depth D of the notch 25 isdefined as the length from the top of the virtual rib 11 without a notch25 to the deepest portion of the notch 25.

Alternatively, as shown in FIG. 8( b), it is possible to structure thenotch 25 so as to have two side surfaces 25 b each extending in thedirection of the intersection P from a point Q on a normal line m of themain part 9 that passes through the mid point of each arc section S inthe arc direction, and a bottom surface 25 c of an arc shape along themain part 9 connecting the two side surfaces 25 b. In this case, as inthe case shown in FIG. 8( a), taking the draft angle into consideration,it is preferable that the angle between the side surface 25 b and thenormal line m be 45 to 48° as viewed from the front. Note that it isalso possible to form the bottom surface 25 c so as to pass through themid point in the height direction of the rib 11. Also in this case, itis preferable that the depth D of the notch be no less than 1.2 mm. Aslong as the smooth removal of the core is ensured, two or more notches25 may be provided in the middle of the arc section S.

As shown in FIG. 9, the arc section S may have a notch 24 as shown inFIG. 5, FIG. 6, or FIG. 7, and a notch 25 as shown in FIG. 8. As shownin FIGS. 8 and 9, it is preferable that the length L (=L₁+L₂) of the arcsection S without a notch be no less than 10 mm.

In the above-described embodiment, the thickness of the interlayer 5 andthe height of the rib 11 are the same; however, they do not necessarilyhave to be the same. For example, it is possible to make the thicknessof the interlayer 5 greater than the height of the rib 11. However, itis preferable that the thickness of the interlayer 5 be slightly greaterthan the height of the rib 11, for example, by 0.3 mm or less.

One example of a method for manufacturing a golf ball having theabove-described structure is explained next with reference to drawings.This manufacturing method, wherein an interlayer is formed from a rubbercomposition, is explained below. FIGS. 10 and 11 show the method formanufacturing a four-piece golf ball comprising the core of FIG. 5.

First, a core is molded. Here, a predetermined amount of non-vulcanizedrubber composition is placed in a mold. As described above, this rubbercomposition comprises a base rubber, a cross-linking agent, a metal saltof unsaturated carboxylic acid, a filler and the like, mixed by aBanbury mixer, rolls, or like mixing equipment. Then, this rubbercomposition is press molded at 130 to 180° C. and a core 3, as shown inFIG. 4, is obtained.

Then, as shown in FIG. 10, an interlayer 5 is formed by press molding.As shown in FIG. 10( a), the mold for the interlayer comprises an uppermold 43 and a lower mold 45, each having a hemispherical depression 41.The depressions 41 of the upper mold 43 and lower mold 45 have the samekind of roughly finished surfaces as those of the molds for the core.Around each depression 41, a plurality of depressions 49 for holdingexcess flow are formed.

As shown in FIG. 10( a), a non-vulcanized rubber composition 61 isinserted into the depression 41 of the lower mold 45, a rubbercomposition 61 is placed on the above-obtained core 3, and the core 3 ispositioned between the upper mold 43 and the lower mold 45.Consequently, as shown in FIG. 10( b), the upper mold 43 and the lowermold 45 are brought into contact. The rubber composition 61 is subjectedto full vulcanization at 130 to 180° C. for 5 to 25 minutes, and pressmolding, to obtain an interlayer 5.

At this time, the rubber compositions 61 placed on the core 3 and in thedepression 41 of the lower mold 45 fill the depressions 13 while beingpressed against the surface of the core 3. As described above, the twoadjacent depressions 13 communicate with each other through notches 24,and therefore the rubber composition spreads throughout each depressionand uniformly fills the space therein. The interlayer 5 may also bemolded by injection molding using, for example, a mold such as thatshown in FIG. 11. In this case, if no notch is provided, it isimpossible to uniformly place the rubber composition in each depression13 without providing a gate for each depression 13. However, byproviding notches 24 to the ribs 11, it is possible to uniformly insertthe rubber composition into the depressions 13 via the notches 24 asdescribed above by inserting the rubber composition even from a singlegate 50 after placing the core 3 into the molds 47 and 48.

When the molding of the interlayer 5 is completed, the core 3 coveredwith the interlayer 5 is removed from the mold. Thereafter, a cover 7 isapplied to the surface of the interlayer 5 by press molding or injectionmolding. Thereafter, an outer cover is applied to the surface of theinner cover by press molding or injection molding in such a manner thatthe cover has predetermined dimples, thus obtaining a golf ball of thepresent embodiment.

As described above, notches 24 are provided in the ribs 11, and the twoadjacent depressions 13 communicate with each other through the notches24. Therefore, the rubber composition 61 spreads throughout thedepressions 13 and uniformly fills the space therein when pressed fromany position on the surface of the core 3. It is thus possible to coverthe core 3 with the interlayer in a single press molding step. As aresult, the manufacturing time can be significantly reduced.

A method for manufacturing a golf ball comprising an interlayer withnotches is explained above. However, a golf ball without notches canalso be manufactured by almost the same method. However, when notchesare not provided, it is necessary to conduct press molding by arrangingthe interlayer material so that it spreads throughout the depressions,or, for injection molding, to provide a plurality of gates correspondingto the depressions.

One embodiment of the golf ball of the present invention is explainedabove. However, the golf ball of the present invention is not limited tothis embodiment, and various modifications can be made as long as theydo not depart from the scope of the invention. For example, three ribsare formed along the great circle drawn around the main part in thepresent embodiment. However, the embodiment of the rib is not limited tothis and the shape, number and location thereof may be appropriatelymodified as long as depressions, to which an interlayer is inserted, canbe formed by the ribs.

EXAMPLES

Examples of the present invention and Comparative Examples are explainedbelow. Here, seven types of golf balls according to the presentinvention are compared with nine types of golf balls according toComparative Examples. The Examples correspond to the embodimentdescribed above. Table 1 below shows the shape of each golf ball, andTable 2 shows the hardness of each component of the golf balls. Thesegolf balls had a diameter of 42.7 mm and a weight of 45.5 g.

TABLE 1 Shape (mm) Diameter Outside Thickness Thickness of Main HeightDiameter of of Inner of Outer Part of Rib Interlayer Cover Cover Example1 33.9 1.8 37.5 1.5 1.1 Example 2 29.9 3.8 37.5 1.5 1.1 Example 3 35.11.2 37.5 1.5 1.1 Example 4 33.1 2.2 37.5 1.5 1.1 Example 5 33.9 1.8 37.51.5 1.1 Example 6 33.9 1.8 37.5 1.5 1.1 Example 7 34.7 1.8 38.3 1.1 0.9Comp. Ex. 1 35.9 0.8 37.5 1.5 1.1 Comp. Ex. 2 29.1 4.2 37.5 1.5 1.1Comp. Ex. 3 34.3 1.6 37.5 1.5 1.1 Comp. Ex. 4 33.1 2.2 37.5 1.5 1.1Comp. Ex. 5 33.9 1.8 37.5 1.5 1.1 Comp. Ex. 6 33.9 1.8 37.5 1.1 1.5Comp. Ex. 7 33.9 1.8 37.5 1.5 1.1 Comp. Ex. 8 35.3 1.8 38.9 0.9 0.9Comp. Ex. 9 33.9 1.8 37.5 — 2.6

TABLE 2 Surface Hardness: D Inner Outer Core Interlayer Cover D Cover DExample 1 56 59 68 57 Example 2 56 58 68 57 Example 3 56 60 68 57Example 4 54 56 58 56 Example 5 56 59 68 57 Example 6 56 59 64 60Example 7 56 59 68 57 Comp. Ex. 1 56 60 68 57 Comp. Ex. 2 56 59 68 57Comp. Ex. 3 56 62 68 57 Comp. Ex. 4 56 60 58 62 Comp. Ex. 5 56 59 68 52Comp. Ex. 6 56 59 58 60 Comp. Ex. 7 56 60 53 51 Comp. Ex. 8 56 60 68 57Comp. Ex. 9 56 60 — 62

Tables 3 and 4 show the compounding ratio (unit:parts by weight) of thematerials for the cores and the interlayers.

TABLE 3 Cores of Examples 1, 2, 3, 5 and 6, and Comparative Examples 1-7Core of and 9; and Interlayer of Core of Core of Comparative InterlayerExample 4 Example 4 Example 7 Example 8 of Example 2 Cis-1,4- 100.00100.00 100.00 100.00 100.00 polybutadiene Zinc oxide 3.00 3.00 3.00 3.003.00 Barium sulfate 26.10 26.90 22.4 21.8 25.40 Antioxidant 0.10 0.100.10 0.10 0.10 Zinc acrylate 25.20 23.20 25.20 25.20 27.40 Dicumylperoxide 1.50 1.50 1.50 1.50 1.50

TABLE 4 Inter-layers of Inter-layers of Inter- Inter- Inter- Examples 1,5 and Example 3, and layer of layer of layer of 6, and ComparativeComparative Examples Comparative Comparative Comparative Examples 2, 5and 6 1, 4, 7 and 9 Example 7 Example 3 Example 8 Cis-1,4- 100.00 100.00100.00 100.00 100.00 polybutadiene Zinc oxide 3.00 3.00 3.00 3.00 3.00Barium sulfate 24.90 24.50 21.00 23.50 20.00 Antioxidant 0.10 0.10 0.100.10 0.10 Zinc acrylate 28.60 29.80 28.60 32.40 29.80 Dicumyl peroxide1.50 1.50 1.50 1.50 1.50

The materials for the inner covers and outer covers are shown below.Table 5 shows the compounding ratio of the materials.

TABLE 5 Inner Cover Outer Cover Example 1 1706:1601 = 1:1 HPC:8150 = 4:1Example 2 1706:1601 = 1:1 HPC:8150 = 4:1 Example 3 1706:1601 = 1:1HPC:8150 = 4:1 Example 4 HPF1000 HPC:8150 = 4:1 Example 5 1706:1601 =1:1 8320:8150 = 3:2 Example 6 1706:1601:8320 = 1:1:1 HPC:8150 = 2:1Example 7 1706:1601 = 1:1 HPC:8150 = 4:1 Comp. Ex. 1 1706:1601 = 1:1HPC:8150 = 4:1 Comp. Ex. 2 1706:1601 = 1:1 HPC:8150 = 4:1 Comp. Ex. 31706:1601 = 1:1 HPC:8150 = 4:1 Comp. Ex. 4 HPF1000 HPC:8150 = 4:3 Comp.Ex. 5 1706:1601 = 1:1 HPC:8150 = 9:1 Comp. Ex. 6 HPF1000 HPC:8150 = 2:1Comp. Ex. 7 1706:8320 = 1:5 HPC:8150 = 9:1 Comp. Ex. 8 1706:1601 = 1:1HPC:8150 = 4:1 Comp. Ex. 9 — HPC:8150 = 4:3

In Table 5, 1706 stands for Himilan 1706 manufactured by Du Pont-MitsuiPolychemicals Co., Ltd., and 1601 stands for Himilan 1601 manufacturedby Du Pont-Mitsui Polychemicals Co., Ltd; HPC stands for HPC AD 1043 (anionomer manufactured by DuPont, HPF stands for HPF 1000 (an ionomermanufactured by DuPont); and 8150 stands for Surlyn 8150 (an ionomermanufactured by DuPont).

Using the golf balls obtained in the Examples and Comparative Examples,which have the above-described structures, hitting tests were conductedusing a hitting robot (manufactured by Miyamae Co., Ltd.: product name“SHOT ROBO V”) with a number 1-wood (1W: manufactured by MizunoCorporation: MP Craft 425, loft angle: 9.5°, shaft: QUAD 6 Butt Standard(shaft length: 45 inches, shaft flex: S)) and sand wedge (SW:manufactured by Mizuno Corporation: MP T Series, loft angle: 56°, Chromeplated, shaft: Dynamic Gold Wedge Flex, shaft length: 35.25 inches), andthe carry distances were measured. Here, the head speed of the 1-woodwas set at 45 m/s, and the head speed of the sand wedge was set at 35m/s. Tests of the feeling when hit were conducted by five amateurs usinga 1-wood. The five amateurs were asked to select either 1: very soft, 2:soft, 3: hard, or 4: very hard, to evaluate the feeling when the ballwas hit and the average value of all values selected was defined as thefeeling value for each Example and Comparative Example. Durability testswere also conducted. In the durability tests, balls were shot from anair gun at 40 m/s, and repeatedly allowed to impact against an ironplate to determine the number of strikes until the balls cracked. Adurability test was conducted using the ball of Example 1, and thenumber of impacts until the ball cracked was set to 100. The relativevalue of each ball was then calculated as a durability index. Table 6below shows the results.

TABLE 6 Feel When Actually 1-Wood: 45 m/s Hit with Wedge Carry Spin a1-Wood Spin Durability (y) (rpm) feeling (rpm) index Example 1 230 24902.2 5810 100 Example 2 229 2460 2.0 5790 97 Example 3 230 2530 2.2 5810102 Example 4 227 2520 1.2 5230 97 Example 5 227 2420 2.6 5310 97Example 6 231 2420 2.6 5030 98 Example 7 231 2280 2.0 5750 94 Comp. Ex.1 226 2630 2.2 5850 101 Comp. Ex. 2 226 2330 2.4 5760 94 Comp. Ex. 3 2262320 2.2 5770 95 Comp. Ex. 4 231 2350 2.0 4480 102 Comp. Ex. 5 225 26601.8 6080 95 Comp. Ex. 6 226 2360 2.4 5890 102 Comp. Ex. 7 221 2690 1.26110 101 Comp. Ex. 8 223 2490 1.8 5790 88 Comp. Ex. 9 230 2410 2.9 481099

As is clear from the results shown in Table 6, Examples 1 to 7 exhibitedexcellent results. However, Comparative Example 1 showed anunsatisfactory backspin reduction effect due to the ribs being tooshort. In contrast, Comparative Examples 2 and 3 showed undesirablylarge reductions in the backspin amount, because the ribs in ComparativeExample 2 were too high and Comparative Example 3 had an undesirablylarge difference in height between the core and the interlayer. Thisreduced the carry distance. One of the reasons for the unsatisfactorycarry distance may be a reduction in the lift force applied to the ball.

The outer cover in Comparative Example 4 was too hard and therefore hada small deformation therein. This caused a small backspin amount when asand wedge was used; therefore, the ball in Comparative Example 4 is notsuited for use in an approach shot. In contrast, the outer cover inComparative Example 5 was too soft. This caused excessive backspin evenwhen a driver was used, resulting in an unsatisfactory carry distance.The outer cover in Comparative Example 6 was unduly thick. This reducedthe rebound resilience and resulted in an unexpectedly short carrydistance. In Comparative Example 7, both the outer cover and the innercover were soft. This reduced the rebound resilience and resulted in anunexpectedly short carry distance. In Comparative Example 8, the totalthickness of the inner cover and the outer cover was too thin, resultingin poor durability. The ball of Comparative Example 9 was hard becauseit had a single-layer cover. This adversely affected the impact feel,and resulted in a small amount of backspin when a sand wedge was used.

As is clear from the results described above, the golf ball of thepresent invention remarkably increases the carry distance and generatesan adequate amount of backspin in an approach shot.

1. A golf ball comprising: a spherical main part; a plurality of ribsformed on the surface of the main part; an interlayer placed indepressions surrounded by the ribs, the interlayer having a hardnessgreater than that of the ribs; an inner cover covering the interlayer;and an outer cover covering the inner cover, the outer cover having ahardness lower than that of the inner cover.
 2. The golf ball accordingto claim 1, wherein the inner cover has a hardness greater than that ofthe interlayer.
 3. The golf ball according to claim 1, wherein the totalthickness of the inner cover and the outer cover is not less than 1.9 mmor more.
 4. The golf ball according to claim 2, wherein the totalthickness of the inner cover and the outer cover is not less than 1.9 mmor more.
 5. The golf ball according to claim 1, wherein the inner coverhas a Shore D hardness of 55 to
 70. 6. The golf ball according to claim1, wherein the outer cover has a Shore D hardness of 54 to 60.